The Use of Linux in an Embedded System
I work for a company that's involved in the design and manufacturing of custom circuits. Historically, we have designed printed circuit boards and boxes that are further integrated by our customers into their systems. Until recently we were rarely involved in what is commonly called systems design and integration. However, times are changing and as we broaden our customer base, we have adapted to meet these needs. Most of the time, we have been able to meet the immediate requirements using DOS and in some cases Windows. This year, we began using Windows 98 to be able to use some of the newer video capabilities, and in one case, Linux stepped in to quickly solve a network connectivity problem. This paper is a brief description of one project that provided the opportunity to get more involved with Linux and the capabilities of this new operating system.
Recently, a customer, who is involved in retail sales and has customarily done their own systems integration, asked us to look at designing a Card Access System to replace their existing system. There were two primary reasons for this. One of these was cost. They felt that by designing a system to specifically meet their requirements, the production cost would be lower than by asking for customization of a product already on the market. The second reason was much more important to them. They wanted to be able to extend this new system and add interfaces into other parts of their overall store system. The store system includes their point-of-sale (POS) controller and a link to a security video system. These aspects of the system design will be discussed in more detail later.
The basic requirements to be met were:
Entry into an area was to be controlled through the use of a credit-card-style key.
If a user did not have their key it would be possible for them to enter the number manually.
Employees could be added or deleted either automatically through the POS system or manually by the store manager.
Access to an area was granted or denied based on the key and the type of area in question.
Configuration of the system could be changed easily on site.
If any area was accessed without a valid key, an alarm was to be sounded.
Alarms could be reset by a manager, or optionally, by any valid key.
Alarm conditions would be sent to the security video system.
Managers would be able to override the system and allow doors to be propped open for extended periods of time.
A must was the capability to query the system and examine activity based on area, time and/or employee.
Although these requirements are fairly basic, three things stood out. The first was they wanted the updates of the access database to occur from the POS system. However, this was something that remained “in the future”. Because of this, the second point was updates would need to be made locally which required an intuitive interface. Finally, the design of the security video system was being designed at the same time so the link to that would also be in the future. Other than having so many points that would remain for in the future, this was quite typical of many of the projects we've done. One other difference was we were to deliver a complete system rather than just the parts.
Because of the distributed nature of the store environment, the decision was made to use an EIA RS-422 type of interface to distribute the system throughout the store. The basic idea was to have several serial lines from the central PC going out to multiple door-access modules on each serial line. Each door-access module would control up to four readers (or doors), see Figure 1. The choice of four readers was based on a typical proximity of doors and the amount of wiring needed for the readers, the solenoids and alarms. Since we had designed card readers and other data entry type of equipment in the past, it was decided to implement the card reader as a wall-mounted panel with an 8051 type device controlling it. Any entry from either the keypad or from a card swipe would be buffered and sent to the door-access module when polled. The door-access module would later forward it on to the central PC when it was polled. The design of the system allows up to ten door-access modules per serial link or a maximum of 40 doors per serial link.
When the project was started, the feeling was that we would use Windows 95/98 as the operating system. This was a natural considering that in addition to the necessary communication it could provide database services through the use of Microsoft Access and an intuitive user interface. However, as the hardware design progressed, it became apparent the programming resources would not be available within an acceptable time frame. When it came time to allocate resources for this job, I was winding down from another job and was asked about how I would approach the design of the user interface. Not being very familiar with Windows programming but having some exposure to the browser interface, I asked if it would be appropriate to switch to Linux and provide the user interface through the use of Apache and an appropriate browser. The design team immediately picked up on the idea and presented it to the customer. When it was pointed out that this implementation would not require the user to be present at the equipment, which is typically crammed into a closet, the customer responded, “Neat.” When asked if they would have objections to implementing it on Linux, their response was, “Cool.” With this, I was on my way.